This invention relates to the electrolytic reduction of hydroxybenzaldehydes to produce the corresponding 1,2-bis(hydroxyphenyl)ethane-1,2-diol (dihydroxybenzoin). More particularly, this invention relates to the electrolytic reductive coupling of hydroxybenzaldehydes by electrolysis in an aqueous alkaline electrolysis medium in an undivided electrolytic cell to produce 1,2-bis(hydroxyphenyl)ethane-1,2-diols.
Electrolytic reductive coupling of hydroxybenzaldehydes to prepare the corresponding 1,2-bis(hydroxyphenyl)ethane-1,2-diols has previously been accomplished in good yields. See, for example, Grimshaw et al, Journal of the Chemical Society (C), 653 (1966). However, each of the methods described in the prior art employed a divided cell. Such cells, of course, suffer from numerous inherent difficulties. For example, a divided cell is inherently more complex than an undivided cell, thereby incurring higher costs in cell construction. In addition, a divided cell exhibits a higher internal resistance than an undivided cell. As a result, efforts to adapt such divided-cell electrolytic methods of preparing 1,2-bis(hydroxyphenyl)ethane-1,2-diols to commercial production on a large technical scale have been severely limited.
While undivided cells do not suffer from the difficulties associated with divided cells, such cells have not been employed for the electrolytic reductive coupling of hydroxybenzaldehydes, apparently because of the general expectation that the well-known anodic oxidative coupling reactions which phenols are prone to undergo would occur to give unwanted by-products, particularly in an alkaline medium.
The difficulties and disadvantages encountered in the prior art processes of electrolytic reductive coupling of hydroxybenzaldehydes are overcome by the discovery that the electrolytic reductive coupling of hydroxybenzaldehydes to yield 1,2-bis(hydroxyphenyl)ethane-1,2-diols is conveniently accomplished in an undivided cell, a decided advance in the state of the art.